The present invention generally relates to materials science and more particularly to an apparatus and method for the identification of the crystalline phase of a crystalline sample and subsequent determination of its crystallographic characteristics.
The physical properties and characteristics of crystalline materials are strongly determined by the arrangement of atoms within the compound or compounds that make up the bulk material. Often, there are many compounds that have the same chemical composition but different crystal structures and will behave differently (e.g. TiO.sub.2 which can exist as three different crystal structures, each having entirely different physical properties). Thus, in order to predict a materials response or behavior, it is important to know the crystal phases that are present.
Although there are only seven basic crystal types, there are over 100,000 different crystal phase variations. If known, the crystal phase of a material provides structural parameters and geometrical relationships necessary for the determination of other material properties. Thus, the crystal phase is an important characteristic of a material that can be valuable.
Generally, for unambiguous phase identification, analysts have needed to resort to very expensive instruments whose sample preparation requirements necessitated the destruction of the sample. These techniques used by these instruments include micro X-ray diffraction (MXRD) and transmission electron microscopy (TEM). Unfortunately, MXRD is a technique that has a limited spatial resolution (&gt;30 micrometers) or requires a synchrotron for higher resolution. Also, MXRD does not form spatial images of the sample so that the spatial relationship of various phases may be visualized. TEM is not limited by resolution like MXRD. The spatial resolution of the TEM is less than 0.1 micrometers and can form images of the material at this resolution or higher so that the spatial relationship between various constituents of the microstructure may be visualized. The great disadvantage to the TEM technique is the extensive and difficult specimen preparation that must be performed and the manual indexing of the diffraction patterns that must be performed. These two disadvantages make TEM a more time consuming and expensive technique. These systems are optimized for texture mapping. These instruments have also required long analysis times (generally several hours) and considerable expertise to provide phase identification. These instruments are also costly, requiring hundreds of thousands of dollars in investment in the capital equipment along with the costs associated with labor for analysis of the data. If the phase to be analyzed existed mainly on the surface of a component, even these more expensive alternatives have been unlikely to produce positive results.
Therefore, for many applications, crystal lattice planes and crystalline phases are typically identified by a procedure known as crystal indexing. For example, a crystal phase identify is typically assumed when performing crystal indexing processes. Given the identity of the crystal phase, mathematical relationships required to calculate the crystal indices are selected. However, many polycrystalline materials have more than one distinct crystal phase. In addition, other materials may have an unknown phase or an unknown combination of phases. Thus, an indexing solution for such a material may be incomplete or erroneous without a proper identification of the crystal phase or phases within the particular specimen.
Automated crystal indexing procedures have enabled researchers, material processors, and manufacturers to obtain valuable microstructure information over a relatively large material area. Generally, such a procedure repetitively bombards selected points of a material specimen with a beam of electrons. The electrons interact with a small volume of the material sample at the selected points, and diffracting crystals cause electron backscatter diffraction patterns or backscattered electron Kikuchi patterns (BEKPs) to form on a screen near the specimen. The BEKPs may be imaged through a video camera and digitized for further processing.
Good-quality BEKPs include a number of intersecting, relatively high intensity bands that are usually referred to as Kikuchi bands or lines. The Kikuchi bands result from electrons being diffracted from various planes in the crystal lattice at the point of bombardment. Goehner and Michael (Goehner, R. P. and Michael, J. R., J. of Research of the NIST, 101, 3, 1996) describe a scanning electron microscope (SEM) with a charge coupled device based detector to permit BEKPs to be collected. An abundance of microstructure information, including the crystal indexing solution, may be obtained by analyzing the various parameters of the Kikuchi bands. Computer-implemented image processing techniques have been developed to analyze Kikuchi bands from BEKPs taken at numerous points on a material sample and to generate displays of the specimen that convey microstructure information.
One tool used to examine microphases is the scanning electron microscope (SEM), an instrument which uses an electron beam to examine materials (from insects to computer chips) at very high magnifications, allowing examination of individual features as small as 1/1000 the diameter of a human hair. SEMs allow determination of the elements present in these tiny regions of a sample, but do not allow unambiguous identification of the crystalline phase those elements comprised.
Adams, Dingley, and Field (U.S. Pat. No. 5,466,934, issued on Nov. 14, 1995) describe an apparatus using an SEM to characterize crystalline defects by comparing backscatter images of a sample with a baseline to detect the defects. The apparatus and method do not, however, determine the crystalline phase of the sample but only defects in the crystalline phase.
Field and Dingley (U.S. Pat. No. 5,557,104, issued on Sep. 17, 1996) describe an apparatus and technique for determining the crystallographic characteristics of a specimen. Field and Dingley use a conventional processing technique utilizing a computational iteration scheme to determine the resultant crystal indexing solution. The indexing solution is calculated a number times using different computation parameters, and a voting algorithm selects the best solution based on a given starting set of possible solutions. Their technique evaluates three-band sets of Kikuchi bands to generate a ranking of the most probable matches to different indexing solutions based on their frequency of occurrence. Unfortunately, this algorithm assumes that the true solution is given in the initial small set of provided possible solutions and only ranks the different possible solutions. No probabilistic measurements or statistical confidence data are included in the analysis. Thus, one must rely on the selected indexing solution without knowing how reliable the data may actually be. Any further analyses or calculations based on an unreliable indexing solution will also be unreliable, and a reviewer may not know that the results are unreliable.
Advantageous would be a method that determines automatically and unambiguously the crystalline phase and crystallographic characteristics of a specimen without the need for extensive sample preparation or sample destruction. Additionally, advantageous would be a method that can unambiguously determine the crystalline phase without knowing a priori what the crystalline phase might be.